Deoxydehydration of vicinal diols and polyols catalyzed by pyridinium perrhenate salts

Article abstract of DOI:10.1039/c7cy01728f

Simple ammonium and pyridinium perrhenate salts were evaluated as catalysts for the deoxydehydration (DODH) of diols into alkenes. Pyridinium perrhenates were found to be effective catalysts at much lower temperatures than those in previous reports, outperforming primary, secondary, and tertiary ammonium salts, while quaternary ammonium salts are effectively inactive. The mechanism of reaction was studied computationally using DFT calculations which indicate that proton shuttling between the ion pair is intrinsic to the mechanism and that the reduction of rhenium by the phosphine occurs before the diol condensation.

Full text of DOI:10.1039/c7cy01728f

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Deoxydehydration of vicinal diols and polyols
catalyzed by pyridinium perrhenate salts†
Cite this: DOI: 10.1039/c7cy01728f
Danny S. Morris,‡a Karlotta van Rees,‡ab Massimiliano Curcio,‡a Mirza Cokoja,c
a
a
Fritz E. Kühn, d Fernanda Duarte
and Jason B. Love
*
Simple ammonium and pyridinium perrhenate salts were evaluated as catalysts for the deoxydehydration
(DODH) of diols into alkenes. Pyridinium perrhenates were found to be effective catalysts at much lower
temperatures than those in previous reports, outperforming primary, secondary, and tertiary ammonium
salts, while quaternary ammonium salts are effectively inactive. The mechanism of reaction was studied
computationally using DFT calculations which indicate that proton shuttling between the ion pair is intrinsic
to the mechanism and that the reduction of rhenium by the phosphine occurs before the diol
condensation.
Received 23rd August 2017,
Accepted 11th October 2017
DOI: 10.1039/c7cy01728f
rsc.li/catalysis
alysts for DODH reactions,5d,6 but are significantly less active
than, for example MeReO3 which has been ascribed to the poor
Introduction
The deoxygenation of oxygen-rich, biomass-derived materials
such as glycerides and carbohydrates could potentially pro-
vide renewable routes to platform chemicals and transporta-
tion fuels, so delivering alternatives to the use of fossil-based
resources.1 One strategy towards this target is the use of cata-
lytic deoxydehydration (DODH),2 a reaction that transforms
vicinal diols, polyols, and carbohydrates into alkenes in the
presence of a reducing agent (eqn (1)).
solubility of the former complexes.7 The majority of these Re cat-
alysts need elevated temperatures (130–250 °C) for turnover and,
with the exception of perrhenate, are toxic8 and not easily recycled.
In 2015, we reported that the toluene-soluble perrhenate
salt of the cation 1 (Chart 1) acts as a catalyst for the epoxida-
tion of alkenes by H2O2 under biphasic conditions.9 Through
microscopic reversibility and the literature precedent above,
we envisaged that 1 should also be able to act as a catalyst
for reductive deoxygenation reactions. As such, with a view to
understanding the use of perrhenate in DODH and the effect
of its attendant cation, we show here that pyridinium
perrhenates such as 1 are effective and selective catalysts for
the DODH of diols. These catalysts operate at significantly
lower temperatures than for previously reported oxo-metal
catalysts and, furthermore, the identity of the cation and its
ability to act as a Brønsted acid is important to the efficacy of
this catalyst system.
(1)
Although some oxo complexes of V and Mo have been
exploited as catalysts for DODH,3 the majority make use of the
redox activity of rhenium. These include low oxidation-state
complexes such as Re2IJCO)10 and BrReIJCO)5,4 but more com-
monly ReVII/ReV oxo complexes such as MeReO3, Cp*/CptttReO3
(Cpttt = But3C5H2), ReO2IIJPPh3)2, and ReOCl3IJPPh3)2.4a,b,5 The
simple perrhenate salts HReO4 (and Re2O7), NaReO4,
(NH4)ReO4, and (Bun4N)ReO4 act as molecular or supported cat-
a EaStCHEM School of Chemistry, University of Edinburgh, David Brewster Road,
Edinburgh EH9 3FJ, UK. E-mail: jason.love@ed.ac.uk; Tel: +44 (0)131 650 4762
b Van't Hoff Institute for Molecular Sciences, University of Amsterdam, Science
Park 904, 1098 XH Amsterdam, The Netherlands
c Chair of Inorganic and Metal Organic Chemistry, Faculty of Chemistry,
Technical University Munich, Lichtenbergstr. 4, 85748 Garching, Germany
d Molecular Catalysis, Faculty of Chemistry and Catalysis Research Center,
Technical University Munich, Lichtenbergstr. 4, Garching, 85748 Germany
† Electronic supplementary information (ESI) available: Full experimental, crys-
tallographic, and computational details. CCDC 1568774–1568780. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/c7cy01728f
‡ These authors contributed equally to this work.
Chart 1 Cation variation in perrhenate salts (Z)ReO4 (Z = cation) used
as catalysts for the deoxydehydration of vicinal diols.
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Results and discussion
Synthesis and evaluation of perrhenate salts as DODH
with previously reported oxo-rhenium catalysts. Experiments
were also carried out to investigate deactivation by the
starting materials and/or products (Table 2, entries 16–18).
While the addition of 5 mol% of Ph3PO or styrene does not
affect the profile of the reaction, water is an inhibitor, with a
10% drop in conversion after 4 h. It should be noted that the
inhibition by water, the low conversions seen in more polar
solvents, and the toxicities of the benzene and chloroform
solvents used to ensure best activity provides a significant
challenge to up-scaling this catalyst system.
In contrast with previous DODH studies which operate at
high reaction temperatures (≫100 °C), these pyridinium
perrhenate catalysts are effective at 80 °C. Significantly, when
the reactions are stirred (in contrast to NMR tube reactions),
complete conversion is seen in under 4 h at 80 °C. However,
reactions carried out below this temperature display a net de-
crease in activity with conversions of 23 and <5% at 60 and 40
°C respectively (Table 2, entries 7 and 8), while increasing the
temperature to 90 °C retains full conversion (Table 2, entry 5).
Alternative reducing agents were evaluated, with H2 and CO
resulting in 10% and 40% conversion, respectively (entries 9 and
10). In the latter case, styrene glycol also acts as the reducing
agent, which was evident by the presence of oxidized styrene gly-
col species. Organic reducing agents such as 1-phenylethanol or
1,2,3,4-tetrahydronapthalene were also investigated, but result
in poor conversions of 19 and 15%, respectively (Table 2, entries
11 and 12). The lack of effective alternative reducing agents to
phosphines as a stoichiometric reductant in this reaction is
clearly a hindrance to its large-scale use.
catalysts
The pyridinium (1–7) and ammonium (8–13) salts of
perrhenate were prepared by liquid–liquid extraction of
perrhenic acid into a solution of the base in toluene. The
solid-state structures of 1,9 2, 3, 4, 6, 9 and 10 were deter-
mined by single-crystal X-ray diffraction and, with the excep-
tion of 1 and 2 show formation of an ion pair with a strong
hydrogen-bonding interaction between the cation and a sin-
gle Re oxo (Fig. 1 and ESI†); in contrast, a C–H–O hydrogen
bond is seen in 2 due to the sterically hindering tert-butyl
groups that flank the pyridinium nitrogen atom, and the
diamido motif in the cation of 1 promotes diffuse N–H and
C–H hydrogen bonding to the perrhenate anion.
Along with the commercially available 14 and 15, com-
pounds 1 to 13 were evaluated as catalysts for the DODH of
styrene diol in CDCl3 at 80 °C (Table 1). Blank tests displayed
that DODH does not take place in the absence of catalyst and
that the substrate slowly acts as a reducing agent (i.e. in the
absence of added reducing agent such as PPh3). Disappoint-
ingly, the quaternary ammonium salts 13 to 15 were ineffec-
tive as catalysts (<5% conversion), whereas the 1°, 2°, and 3°
ammonium salts 8 to 12 gave only moderate conversion (30–
65%); in contrast, the pyridinium salts 1 to 7 afforded high
conversion. While 2 results in the highest conversion after 4
h at 80 °C, it displayed poor solubility in CDCl3 so its concen-
tration during the reaction is likely imprecise. As a result, the
reaction conditions were optimized using 3 (Table 2). Al-
though the TOFs at 80 °C for 1 to 7 are low (1.2 to 3.2 h−1),
they are comparable to previous reports and 2 to 3× greater
than the other perrhenate salts reported here.
Catalyst scope
The use of different solvents has a significant impact on
reaction yield. Reactions conducted in benzene and chloro-
form result in complete conversion of the diol (entries 1 and
6) whereas more polar tetrahydrofuran, acetonitrile, and pyri-
dine result in moderate to low conversion (Table 2, entries 2–
4). It is likely that these latter solvents coordinate to rhenium
to inhibit reaction, as observed for CptttReO3.5c It is possible
to decrease the catalyst loading from 10 to 2.5 mol% without
significant decrease of yield (Table 2, entries 13–15), there-
fore a loading of 5 mol% was adopted to compare this system
Following these optimization studies, the scope of the reac-
tion was investigated (Table 3). The aromatic vicinal diol sty-
rene glycol (s1) produces a quantitative yield of styrene (p1)
without polymerization of the product (Table 3, entry 1),
whereas a strongly electron-donating methoxy group (s2), or
strongly electron-withdrawing nitro group (s4) decreases both
activity and selectivity towards alkene formation (Table 3,
entries 2 and 4); the effect is more pronounced for the
nitro group. In contrast, using bromide (s3) as a weakly
deactivating group provides a quantitative yield of styrene
(Table 3, entry 3). No change in activity and selectivity for al-
kene is seen when the size of the conjugated system is in-
creased by substituting phenyl for naphthalene (s5) (Table 3,
entry 5). Although the same activity and selectivity for alkene
is seen for meso-hydrobenzoin (s6), this substrate also forms
trace benzaldehyde due to C–C bond cleavage (Table 3, entry
6); similar oxidative cleavage has been observed to a higher
degree using MeReO3 as a catalyst.10 The functionalized es-
ter, (+)-diisopropyl L-tartrate (s7) is converted with excellent
yield (Table 3, entry 7). The alkyl ether, 1,4-anhydroerythritol
(s8), requires elevated temperature for the reaction to pro-
ceed and results in 51% yield of 2,5-dihydrofuran (p8). The
linear aliphatic terminal diols (s9 and s10) also require
Fig. 1 Solid-state structures of 2 (left) and 3 (right). For clarity, all
hydrogen atoms, except those involved in hydrogen-bonding, are
omitted (displacement ellipsoids are drawn at 50% probability).
Selected bond lengths (Å): 2: Re1–O1 1.703(4); Re1–O2 1.723(4); Re1–
O3 1.703(4); C2–O1 3.312. 3: Re1–O1 1.736(2); Re1–O2 1.714(2); Re1–O3
1.710(2); Re1–O4 1.716(2); N1–O1 2.754.
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Table 1 Catalytic DODH of styrene glycol catalyzed by 1 to 15
Entry
Catalyst
Load (mol%)
Time (h)
Conv.a (%)
TOF (h−1
)
pKa
1
2
3
4
5
6
7
8
1
2
3
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
16
16
16
16
16
16
16
16
>99
>99
>99
>99
37
39
64
21
1.3
1.3
1.3
1.3
0.5
0.5
0.8
0.3
0.3
—
3.2
1.5
2.0
2.5
1.2
3.0
1.0
2.0
—
4.4
6.7
0.5
10.6
9.3
8.5
—
4
10
11
12
13
14
15
2
3
4
5
6
9
16
16
21
<5
—
10
11
12
13
14
15
16
17
18
9.2
4.4
6.7
0.5
3.3
2.8
−0.4
10.1
5.1
1/2/4
1/2/4
1/2/4
1/2/4
1/2/4
1/2/4
1/2/4
1/2/4
21/39/63
12/20/29
17/26/38
18/30/50
10/15/24
22/36/59
5/10/16
18/27/41
7
8
9
a
Conversion was determined by 1H-NMR spectroscopy using Ph3CH as an internal standard.
Table 2 DODH of styrene glycol to styrene using lutidinium perrhenate 3 as catalyst under a variety of conditions
Entry
Reducing agent
Solvent
T [°C]
Time (h)
Cat. (mol%)
Conv.a (%)
1
2
3
4
5
6
7
8
Ph3P
Ph3P
Ph3P
Ph3P
Ph3P
Ph3P
Ph3P
Ph3P
H2, 1 bar
CO, 1 bar
1-Phenyl ethanol
1,2,3,4-C10H12
Ph3P
Ph3P
Ph3P
Ph3P
Ph3P
C6H6
THF
80
80
80
80
90
80
60
40
80
80
80
80
80
80
80
80
80
80
16
16
24
16
16
16
16
16
16
24
16
16
4
5
5
5
5
5
5
5
5
5
5
5
5
10
5
2.5
5
5
5
>99
65
<5
0
>99
>99
23
<5
10
40
19
15
39
CH3CN
Pyridine
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
9
10
11
12
13
14
15
16b
17c
18d
4
4
4
4
33
28
32
32
Ph3P
4
22
a
Reaction conditions: 0.560 M styrene glycol, 0.028 M 3 (5 mol%), 0.610 M reducing agent. Conversion was determined by 1H-NMR
spectroscopy using Ph3CH as an internal standard. b Addition of 0.028 M Ph3PO. c Addition of 0.028 M styrene. d Addition of 0.028 M water.
elevated temperature to give moderate conversions compara-
ble to literature (Table 3, entries 9 and 10).
sion led us to compare the observed conversions with nucleo-
philicity (N)12 and Lever electronic parameters (LEP)13 of the
neutral amine/pyridine. While no correlation is seen with N,
good correlation between LEP and close analogues of the pyr-
idines 3 and 4 and amines 10 and 15 is seen (Fig. S28†). This
suggests that the nature of the neutral amine or pyridine has
mechanistic consequence, e.g. through coordination to an
intermediate (see later). Similar effects were seen in DODH
catalysis using MeReO3 with the addition of pyridine or bipy
resulting in lower product yields,14 but contrasts to previous
evidence that simple ammonium perrhenates are the most
active, yet least selective, for alkene formation.14 Besides this
apparent electronic parameter effect, there is also a decrease
in activity going from o-tBu-substituted 2 to o-Me-substituted
3 (Table 1). Water inhibition could also play a role, as all of
Evaluation of the mechanism of DODH by perrhenate salts
The relationship between the nature of the cation and cata-
lyst activity under the optimized conditions was probed. The
clear requirement of a protic cation led us to study the effect
of its pKa and related electronic and steric effects. While the
range of pyridinium cations 2 to 7 spans pKa values between
−0.4 and 6.7, no clear relationship with their activity is seen
(Fig. S26 and S27†). For the ortho-substituted pyridinium
ions, there is an increase in catalytic activity going from Me
< OMe < F which is allied to an increase in Hammett sigma
constant.11 This possible electronic correlation with conver-
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Table 3 Scope of DODH of diols under optimized conditions catalysed by the lutidinium perrhenate 3
Entry
1
Substrate
Product
Time [h]
4
Conv. [%]
TOF [h−1
5.0
]
Yielda [%]
S1
P1
>99
>99IJ74)b
2
3
4
S2
S3
S4
P2
P3
P4
4
4
80
4.0
5.0
0.7
50
>99
55
>99
22
16
5
6
S5
S6
P5
P6
4
4
>99
5.0
4.9
>99
c
—
98
7
S7
S8
P7
P8
16
16
>99
77
1.3
1.0
>99
8d
51
9d
S9
P9
16
16
41
0.5
0.3
28
21
10d
S10
P10
—
e
a
Reaction conditions: 0.560 M styrene glycol, 0.028 M 3 (5 mol%), 0.610 M Ph3P, 0.5 mL chloroform solvent, 90 °C. Conversion and NMR yield
b
c
d
were determined by 1H NMR spectroscopy using Ph3CH as an internal standard. Isolated yield. Substrate not miscible. At 140 °C.
Conversion was not measured due to poor substrate solubility.
e
these perrhenate salts have some solubility in water, but as
no direct relationship is seen a combination of these factors
may be operating.
The general mechanism of the DODH of diols by oxo-
rhenium complexes has been examined,2c,4b,6d,10,15 and con-
sensus exists in that it proceeds through three steps: (1) re-
duction of rheniumIJVII) to rheniumIJV); (2) condensation of the
vicinal diol and (3) oxidative extrusion of the alkene and cata-
lyst regeneration. To elucidate the mechanism of this process
stoichiometric reactions were targeted. No reaction between 3
and styrene glycol occurs and instead heating triggers
substrate oxidation to phenylglyoxal and 2-hydroxy-1-
phenylethanone along with the production of styrene. Inter-
estingly, styrene formation is not concomitant with diol oxi-
dation, but delayed (Fig. S30†), which suggests that an initial
reduction to a catalytically active rhenium species occurs. To
support this, it is found that heating 3 with PPh3 results in
phosphine consumption and the slow formation of Ph3PO
along with a black, unidentifiable material which is ascribed
to excessive rhenium reduction.2c,6e,10
The reaction mechanism was investigated computationally
using density functional theory (DFT) at the SMD-B3LYP-
D3IJBJ)/Def2QZVP//B3LYP-D3IJBJ)/6-31G+(d) level of theory (see
ESI† for computational details). The preferred pathway pro-
ceeds via reduction of ReIJVII) to ReIJV) followed by diol conden-
Fig. 2 Structures and relative free energies (ΔG, in kcal mol−1; selected
distances in Å) of stationary points involved in the catalytic cycle for
sation (path A, Fig. 2) with the route involving initial coordi-
nation of the diol to perrhenate (path B, Fig. S32†) and its
the DODH reaction of styrene diol by 3. Energy values were calculated
subsequent reduction much higher in energy. In both cases,
at the SMDIJCHCl3)B3LYP-D3IJBJ)/Def2QZVP//B3LYP-D3IJBJ)/6-31G+(d)
reduction of ReIJVII) to ReIJV) is the rate-determining step. In
level of theory relative to the separated species.
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i.e. NH4 > 2,6-Me2py > 2,6-tBu2py, the products of elimina-
tion E from the catalytic pathway are of significantly lower
energy, indicating that access to this off-cycle should be lim-
ited (Fig. S32†). As such, we attribute the differences seen in
the catalytic reactivity to the variation in solubility of these
complexes which, in the case of the ammonium salt 15,
would limit the availability of the perrhenate salt in solution.
Conclusions
We have shown that pyridinium perrhenates act as catalysts for
the deoxydehydrogenation of diols to alkenes at significantly
lower temperatures than other perrhenate salts and rhenium
oxo complexes; in the latter cases, these complexes often de-
compose to poorly active perrhenate salts during the reaction.
The supramolecular interactions between the cation and the
perrhenate anion are important, not only in stabilizing reaction
intermediates and transition states but also in the ability of the
cation to act as a Brønsted acid in a proton-shuttling mecha-
nism. Furthermore, enhancing the solubility of the catalyst in
hydrophobic solvents is important to its efficacy. It will be
interesting to see if this supramolecular ion pair strategy can
be applied to other oxo-anions and their chemistry, especially
for compounds of earth-abundant metals. Whilst the DODH re-
actions catalysed by pyridinium perrhenates occur at a lower
temperature than previous examples, the use of non-polar,
toxic solvents and expensive reducing agents means that alter-
natives must be sought before these catalysts are applicable to
large-scale reactions. Even so, the simplicity in modifying the
pyridinium perrhenates described here along with the potential
for their heterogenisation on suitable anion-exchange resins
could overcome some of these challenges.
Fig. 3 Comparison of the structures and relative free energies (ΔG, in
kcal mol−1) for the catalytic DODH reaction of styrene diol by 3 (black)
and 15 (red) proceeding via reduction, condensation and elimination
(path A). Energy values were calculated at the SMDIJCHCl3)B3LYP-
D3IJBJ)/Def2QZVP//B3LYP-D3IJBJ)/6-31G+(d) level of theory relative to
the separated species. The production of off-cycle products as a result
of proton-shuttling and base coordination is shown.
path A, the first step corresponds to reduction of the metal
and has an activation energy of 30.1 kcal mol−1 relative to the
independent species. The transition state (TS) structure for
this step demonstrates that proton shuttling between the cat-
ion and the anionic rhenium species is important. Following
this step, dissociation of OPPh3 affords the trigonal planar
−
ReO3 and subsequent addition of the diol to ReIJV) via TS2
(22.6 kcal mol−1) results in tetra-coordinated C; the associ-
ated barrier for this step from the intermediate B is similar
to that computed for the activation of ethylene glycol by
MeReO3 (29.3 kcal mol−1).15 Elimination of water occurs via TS3
and is the first exothermic process along the path (−13.3 kcal
mol−1). Here the hydrogen atom being transferred is located
almost halfway between the hydroxide and the oxygen atom
of the phenyl glycol, which is stabilized by the lutidinium cat-
ion by hydrogen bonding. Finally styrene extrusion further
drives the reaction downhill allowing regeneration of the catalyst.
Despite the differences observed experimentally for am-
monium (15) and lutidinium (3) catalysts, similar energetics
were found for both systems and suggests that electronic ef-
fects are not the origin of the differences in activity. To probe
this further, the coordination of the neutral amine/pyridine
to intermediate D (as a consequence of proton transfer,
PSoff-cycle) was evaluated (Fig. 3). Modelling this pathway
shows an activation energy very similar to the canonical path-
way but with a significant decrease in free energy when
forming the neutral complex, −28.4 kcal mol−1 for ammo-
nium and −16.6 kcal mol−1 for lutidinium. While this poten-
tial off-cycle correlates with the LEP effects seen above, in
that the stability of the neutral product ReIJOH)IJO)IJdiol)IJL) (L
= amine or pyridine) decreases with increasing steric bulk,
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
We thank the University of Edinburgh and the EPSRC
CRITICAT Centre for Doctoral Training (Ph.D. studentship to
M.C.; Grant code EP/L016419/1) for financial support.
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